Major branch instructions with transactional memory

ABSTRACT

Major branch instructions are provided that enable execution of a computer program to branch from one segment of code to another segment of code. These instructions also create a new stream of processing at the other segment of code enabling execution of the other segment of code to be performed in parallel with the segment of code from which the branch was taken. In one example, the other stream of processing starts a transaction for processing instructions of the other stream of processing.

BACKGROUND

An aspect of the invention relates, in general, to computer programs,and in particular, to branch instructions used within computer programs.

A computer program (a.k.a., program or application) is a sequence ofinstructions written to perform a specific task within a computingenvironment. Each instruction performs a particular operation, such asadd, subtract, move, shift, compare, store, and load, to name just a fewexamples. The instructions are fetched by a processor in sequentialorder, unless a particular instruction directs the processing tocontinue at a different location. For instance, instead of fetching thenext sequential instruction, a branch instruction may direct theprocessor to fetch an instruction at a different location in theprogram. On average, a program has a branch every four to fiveinstructions.

In some computer micro-architectures, an instruction is executed tocompletion before the next instruction begins. However, in othermicro-architectures, pipelining is used to increase the number ofinstructions that can be concurrently executed. In pipelining, theprocessing of an instruction is split into a plurality of independentsteps, such as fetch, decode, dispatch, issue, execute and complete, aswell as potentially others, which are executed in a pipeline. As thefirst step of the instruction (e.g., fetch) completes in the pipeline,then another instruction may begin, and so forth.

Pipelining also facilitates the execution of instructions out-of-orderfrom their position in the program. For instance, the issue step hasassociated therewith an issue queue, which receives instructionsin-order of the program. The issue queue is responsible for holding eachinstruction until the data (e.g., operands) needed by the instruction isavailable, and then it issues the instruction for execution. If the datafor one instruction is available prior to the data for anotherinstruction, then the one instruction can be sent out for executionbefore the other instruction (out-of-order processing), even if theother instruction is first in the program. For example, if instructionsA, B and C are to be executed, where A is the oldest and C is theyoungest, if B is dependent on A, and C is not dependent on A or B, thenthe instructions can be issued in the order of A, C, B, therebyutilizing the pipeline at a higher rate and allowing instructions to beprocessed faster.

The out-of-order processing, however, is constrained by an out-of-orderwindow/queue of a finite size in which instructions may be processedout-of-order. The issue window is typically 100 or less instructions andthe reorder window, which re-orders the instructions back into itsoriginal order, is typically 200 or less instructions, where the 100 isa subset of the 200.

BRIEF SUMMARY

Shortcomings of the prior art are overcome and advantages are providedthrough the provision of a computer program product for executing amachine instruction. The computer program product includes a computerreadable storage medium readable by a processing circuit and storinginstructions for execution by the processing circuit for performing amethod. The method includes, for instance, obtaining, by a processor, amachine instruction for execution, the machine instruction being definedfor computer execution according to a computer architecture, the machineinstruction including: at least one opcode field for specifying anopcode, the opcode defining a branch operation; and at least one fieldfor determining a target location in which to branch; and executing themachine instruction in a first stream of processing, the executingincluding: starting a second stream of processing, the second stream ofprocessing having a relationship with the first stream of processing;and starting a transaction for the second stream of processing, thetransaction to include execution of one or more instructions beginningat the target location defined using the at least one field of themachine instruction, the one or more instructions being out ofsequential execution order from one or more instructions being processedby the first stream of processing, the first stream of processingcontinuing to process one or more instructions subsequent to the machineinstruction; and wherein execution of the second stream of processing isspeculative in that at least one instruction of the one or moreinstructions of the second stream of processing is executed even thoughreaching the at least one instruction of the second stream of processingby processing of the first stream is tentative, and wherein updating ofarchitected state by one or more instructions of the second stream ofprocessing is performed based on the first stream of processing reachinga predefined point in processing.

Methods and systems relating to one or more aspects of the presentinvention are also described and claimed herein. Further, servicesrelating to one or more aspects of the present invention are alsodescribed and may be claimed herein.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

One or more aspects of the present invention are particularly pointedout and distinctly claimed as examples in the claims at the conclusionof the specification. The foregoing and objects, features, andadvantages of one or more aspects of the invention are apparent from thefollowing detailed description taken in conjunction with theaccompanying drawings in which:

FIG. 1A depicts one embodiment of a computing environment to incorporateand use one or more aspects of the present invention;

FIG. 1B depicts another embodiment of a computing environment toincorporate and use one or more aspects of the present invention;

FIG. 1C depicts further details of the memory of FIG. 1B, in accordancewith an aspect of the present invention;

FIG. 2A depicts one example of an existing design of an instructionpipeline;

FIG. 2B depicts one example of another design of an instructionpipeline, in accordance with an aspect of the present invention;

FIG. 3 depicts an example of a portion of a program that includes one ormore branches, in accordance with an aspect of the present invention;

FIG. 4A depicts one example of a format of a generic branch instruction,in accordance with an aspect of the present invention;

FIG. 4B depicts another example of a format of a generic branchinstruction, in accordance with an aspect of the present invention;

FIG. 5 depicts one example of logic associated with processing majorbranches, in accordance with an aspect of the present invention;

FIG. 6 depicts one example of logic associated with processing majorbranches with transactional memory, in accordance with an aspect of thepresent invention;

FIG. 7 depicts one embodiment of a computer program productincorporating one or more aspects of the present invention;

FIG. 8 depicts one embodiment of a host computer system to incorporateand use one or more aspects of the present invention;

FIG. 9 depicts a further example of a computer system to incorporate anduse one or more aspects of the present invention;

FIG. 10 depicts another example of a computer system comprising acomputer network to incorporate and use one or more aspects of thepresent invention;

FIG. 11 depicts one embodiment of various elements of a computer systemto incorporate and use one or more aspects of the present invention;

FIG. 12A depicts one embodiment of the execution unit of the computersystem of FIG. 11 to incorporate and use one or more aspects of thepresent invention;

FIG. 12B depicts one embodiment of the branch unit of the computersystem of FIG. 11 to incorporate and use one or more aspects of thepresent invention;

FIG. 12C depicts one embodiment of the load/store unit of the computersystem of FIG. 11 to incorporate and use one or more aspects of thepresent invention; and

FIG. 13 depicts one embodiment of an emulated host computer system toincorporate and use one or more aspects of the present invention.

DETAILED DESCRIPTION

Programs are written in code (e.g., source code), which includes theindividual instructions to be executed. There are segments (alsoreferred to as portions, regions, etc.) of code which are not dependenton each other, however, the starting/entry points of these segments areseparated by greater than the hardware out-of-order window. Since theyare separated, as per the number of executed instructions between thetwo points, by greater than the out-of-order window, existing branches,referred to herein as minor branches, cannot be reached in a timelymanner within the out-of-order window of the first segment to executeinstructions of the second segment out-of-order with respect to thefirst segment. To address this, major branches are provided, inaccordance with an aspect of the present invention, to branch from onesegment of code to another segment of code to allow a largerout-of-order window than what can be seen by hardware alone (i.e., theamount of code that can be spanned to enter the out-of-orderwindow/queue is larger, although the out-of-order window/queue remainsthe same physical size (e.g., supporting 100 instructions; the 100instructions coming from multiple threads of instruction, as discussedbelow.)).

In accordance with an aspect of the present invention, major branchinstructions are provided that enable execution to branch from onesegment of code to another segment of code. These instructions create anew stream or thread of execution at the other segment of code enablingexecution of the other segment of code to be in parallel with thesegment of code from which the branch was taken.

In one example, execution of the new stream is speculative in that it isnot known whether processing of the first stream will actually reach theinstructions of the new stream (i.e., the first stream reaching the newstream is tentative). It is possible that one or more instructions ofthe new stream are executed, but processing of the first stream takes adifferent path and does not reach the new stream.

One embodiment of a computing environment to incorporate and use one ormore aspects of the present invention is described with reference toFIG. 1A. A computing environment 100 includes, for instance, a processor102 (e.g., a central processing unit), a memory 104 (e.g., main memory),and one or more input/output (I/O) devices and/or interfaces 106 coupledto one another via, for example, one or more buses 108 and/or otherconnections.

In one example, processor 102 is a z/Architecture® processor which ispart of a System z® server offered by International Business MachinesCorporation (IBM®), Armonk, N.Y. The System z® server implements thez/Architecture®, offered by International Business Machines Corporation,which specifies the logical structure and functional operation of thecomputer. One embodiment of the z/Architecture® is described in an IBM®publication entitled, “z/Architecture Principles of Operation,” IBM®Publication No. SA22-7832-08, Ninth Edition, August, 2010, which ishereby incorporated herein by reference in its entirety. In one example,the processor executes an operating system, such as z/OS®, also offeredby International Business Machines Corporation. IBM®, z/Architecture®and z/OS® are registered trademarks of International Business MachinesCorporation, Armonk, N.Y., USA. Other names used herein may beregistered trademarks, trademarks, or product names of InternationalBusiness Machines Corporation or other companies.

Another embodiment of a computing environment to incorporate and use oneor more aspects of the present invention is described with reference toFIG. 1B. In this example, a computing environment 120 includes, forinstance, a native central processing unit 122, a memory 124, and one ormore input/output devices and/or interfaces 126 coupled to one anothervia, for example, one or more buses 128 and/or other connections. Asexamples, computing environment 120 may include a PowerPC® processor, apSeries® server or an xSeries® server offered by International BusinessMachines Corporation, Armonk, N.Y.; an HP Superdome with Intel ItaniumII® processors offered by Hewlett Packard Co., Palo Alto, Calif.; and/orother machines based on architectures offered by IBM®, Hewlett Packard,Intel, Sun Microsystems or others. PowerPC®, pSeries® and xSeries® areregistered trademarks of International Business Machines Corporation,Armonk, N.Y., USA. Intel® and Itanium II® are registered trademarks ofIntel Corporation, Santa Clara, Calif.

Native central processing unit 122 includes one or more native registers130, such as one or more general purpose registers and/or one or morespecial purpose registers used during processing within the environment.These registers include information that represents the state of theenvironment at any particular point in time.

Moreover, native central processing unit 122 executes instructions andcode that are stored in memory 124. In one particular example, thecentral processing unit executes emulator code 132 stored in memory 124.This code enables the computing environment configured in onearchitecture to emulate another architecture. For instance, emulatorcode 132 allows machines based on architectures other than thez/Architecture®, such as PowerPC® processors, pSeries® servers, xSeries®servers, HP Superdome servers or others, to emulate the z/Architecture®and to execute software and instructions developed based on thez/Architecture®.

Further details relating to emulator code 132 are described withreference to FIG. 1C. Guest instructions 150 comprise softwareinstructions (e.g., machine instructions) that were developed to beexecuted in an architecture other than that of native CPU 122. Forexample, guest instructions 150 may have been designed to execute on az/Architecture® processor 102, but instead are being emulated on nativeCPU 122 (which may be, for example, an Intel® Itanium II® processor). Inone example, emulator code 132 includes an instruction fetching unit 152to obtain one or more guest instructions 150 from memory 124, and tooptionally provide local buffering for the instructions obtained. Italso includes an instruction translation routine 154 to determine thetype of guest instruction that has been obtained and to translate theguest instruction into one or more corresponding native instructions156. This translation includes, for instance, identifying the functionto be performed by the guest instruction and choosing the nativeinstruction to perform that function.

Further, emulator 132 includes an emulation control routine 160 to causethe native instructions to be executed. Emulation control routine 160may cause native CPU 122 to execute a routine of native instructionsthat emulate one or more previously obtained guest instructions and, atthe conclusion of such execution, return control to the instructionfetch routine to emulate the obtaining of the next guest instruction ora group of guest instructions. Execution of the native instructions 156may include loading data into a register from memory 124; storing databack to memory from a register; or performing some type of arithmetic orlogic operation, as determined by the translation routine.

Each routine is, for instance, implemented in software, which is storedin memory and executed by the native central processing unit 122. Inother examples, one or more of the routines or operations areimplemented in firmware, hardware, software or some combination thereof.The registers of the emulated processor may be emulated using registers130 of the native CPU or by using locations in memory 124. Inembodiments, the guest instructions 150, native instructions 156 andemulator code 132 may reside in the same memory or may be disbursedamong different memory devices.

As used herein, firmware includes, e.g., the microcode, millicode and/ormacrocode of the processor. It includes, for instance, thehardware-level instructions and/or data structures used inimplementation of higher level machine code. In one embodiment, itincludes, for instance, proprietary code that is typically delivered asmicrocode that includes trusted software or microcode specific to theunderlying hardware and controls operating system access to the systemhardware.

In one example, a guest instruction 150 that is obtained, translated andexecuted is a branch instruction described herein. The instruction,which is a z/Architecture® instruction in this example, is fetched frommemory, translated and represented as a sequence of native instructions156 (e.g., PowerPC®, pSeries®, xSeries®, Intel®, etc.) which areexecuted.

In another embodiment, one or more of the instructions are executed inanother architecture environment, including, for example, anarchitecture as described in the “Intel® 64 and IA-32 ArchitecturesSoftware Developer's Manual Volume 1,” Order No. 253665-022US, November2006; “Intel® 64 and IA-32 Architecture Software Developer's ManualVolume 2A,” Order No. 253666-022US, November 2006; the “Intel® Itanium®Architecture Software Developer's Manual Volume 1,” Doc. No. 245317-005,January 2006; the “Intel® Itanium® Architecture Software Developer'sManual Volume 2,” Doc. No. 245318-005, January 2006; and/or the “Intel®Itanium® Architecture Software Developer's Manual Volume 3,” Doc. No.245319-005, January 2006; each of which is hereby incorporated herein byreference in its entirety.

In one embodiment, a processor can be executed in simultaneousmultithreading (SMT) mode or single thread mode. In simultaneousmultithreading mode, the processor can simultaneously execute multipleindependent threads of execution. A thread, as described in furtherdetail below, may be used in executing one or more instructions of acomputer program.

As described above, to process the instructions of a computer program,an instruction pipeline is used, examples of which are described withreference to FIGS. 2A-2B. Referring to FIG. 2A, in an existing design,the following steps of each instruction are performed in-order 200:instruction fetch (I-Fetch), instruction decode, group (e.g., groupingof instructions for how they are to be tracked through the pipeline),and instruction dispatch. That is, if Instruction A is first in theprogram, followed by Instruction B, then fetch, decode, group, anddispatch of Instruction A are performed prior to those steps ofInstruction B. However, the steps of Instruction B may be started priorto completion of the steps of Instruction A; e.g., a fetch ofInstruction B can commence after a fetch of Instruction A is complete,but before decode of Instruction A is complete. Each of these steps mayalso work on more than a single instruction in parallel.

The steps of instruction issue, instruction execution and reordering,however, may be performed out-of-order 202. For instance, Instruction Bmay be executed before Instruction A, but not completed. The completionof the instruction (e.g., checkpointing) is again performed in-order204, such that, for instance, Instruction A is to complete beforeInstruction B. One reason to force in-order completion is to allowprecise exception detection and reporting for instructions which behaveillegally with respect to the architecture.

In accordance with an aspect of the present invention, another design ofthe instruction pipeline is provided in which more of the processing canbe performed out-of-order. For instance, as shown in FIG. 2B, theinstruction fetch, decode, group, dispatch, issue, execute and reordercan be performed out-of-order 250, while the complete step is performedin-order 252. This design is used, for instance, in the processing ofmajor branch instructions, as described herein. (Although variousinstruction steps are described herein, more, less or different stepsmay be used in other examples.)

One type of branch, referred to herein as a minor branch, defines thenext instruction in the program to be executed. This next instruction isnot the next instruction in sequential order of the program, but isstill part of the processing stream (e.g., thread) currently beingexecuted. Contrasted with minor branches is another type of branch,referred to as major branches, provided in accordance with an aspect ofthe present invention. A major branch defines a code point (targetaddress) which is not next in sequential program order and will not bepart of the processing stream currently being executed, but instead,will be executed as part of a new processing stream (e.g., thread). Thistype of branch is further described with reference to FIG. 3.

Referring to FIG. 3, one example of a computer program 300 (or a portionthereof) is depicted, which includes a plurality of instructions. Thefirst instruction of the computer program is Instruction A 302.Processing begins with Instruction A and proceeds to Instruction B 304,Instruction C 306 and Instruction D 310 in sequential order. In thisexample, Instruction D is a major branch Open instruction to target Y340. The major branch Open instruction is executed causing a secondstream (referred to herein as a child stream) to be started/opened (forexample, a second thread on a SMT processor, referred to herein as achild thread, is activated). The first stream (referred to herein as theparent stream) continues to process Instruction E 312, Instruction F 314and Instruction G 320. Instruction G branches to Target X 322, at whichInstruction M 324 is processed, followed by Instructions N 326 and O328. These instructions are also processed as part of the first stream,since the branch at Instruction G is a minor branch and does not causeanother stream of processing to be started/opened.

In accordance with an aspect of the present invention, the processing ofthe parent stream (i.e., one or more of Instructions E, F, G, M, N andO) subsequent to the major branch Open is parallel to the processing ofthe child stream (i.e., one or more of instructions: T 342, U 344, and V346).

Instruction O, in this example, is a major branch Close instruction,which branches to Target Y 340. The close instruction denotes that theparent thread has caught up to the target address of the major branchOPEN. At this point, the child thread becomes the parent thread, and thethread resources assigned to what was the parent thread are nowavailable for further processing (e.g., as another child thread).

The number of threads available depends on the architecture of theprocessor. For instance, for an SMT-2 processor, there are two threadsavailable, and therefore, one child (a.k.a., assist) thread can bestarted. For an SMT-4 processor, there are four threads available, andtherefore, three child threads can be started allowing for furtherout-of-order processing.

In one embodiment, the instructions of the child thread do not completeuntil the instructions of the parent thread have completed, sinceinstructions are to complete in-order in the example pipeline designsdescribed herein. At the time the instructions of the parent threadcheckpoint (e.g., save its state, such as architectural state), thechild thread becomes the parent thread and the instructions which werein what was defined as the child thread are now allowed to checkpoint.

In accordance with an aspect of the present invention, any existing ornew branch instruction of an instruction set architecture can be definedas a major branch instruction, as well as or in lieu of a minor branchinstruction. For instance, many architectures currently have a number ofbranch instructions that perform various branch operations. Each ofthese branch instructions (or a subset thereof) can be implemented as amajor branch (OPEN/CLOSE) and/or a minor branch instruction. Toaccomplish this, the opcode or other field of the branch instructiondefines the type of the branch instruction as either a major branch(e.g., major branch Open or major branch Close) or a minor branch.

For instance, a branch instruction could have one of three differentopcodes: minor branch opcode, major branch Open opcode or major branchClose opcode. Although the three types of branch instructions would havedifferent opcodes, the format of the instruction is the same, as well asthe branch operation. The difference lies in how the instruction isprocessed. For example, its pipeline processing is different, its targetaddress may be different, and its processing as far as being part of theinitial thread or another thread depends on the type of branchinstruction.

In a further example, instead of different opcodes, a field is added tothe instruction (e.g., a separate field from the other fields of theinstruction or an add-on to one of the existing fields, such as theopcode field or another field) that indicates the type of branchinstruction. For instance, a 2-bit field may be used that has thefollowing meaning, in one particular example:

00—Minor branch; instruction behaves as it has

01—Unused

10—Major branch Close

11—Major branch Open

Embodiments of formats of a generic branch instruction are depicted inFIGS. 4A-4B. As shown in FIG. 4A, a branch instruction 400 includes atleast one opcode field 402 including an opcode indicating the particularbranch instruction, as well as the type of branch instruction (majorOpen, major Close, minor); and a target address field 404 used todetermine the location to which the branch instruction is to jump.

In a further example, a branch instruction 420 (FIG. 4B) includes atleast one opcode field 422 including an opcode indicating the particularbranch instruction, a format field 424 designating the type of branchinstruction (e.g., major Open, major Close, minor), and a target addressfield 426. Format field 424 may be a separate field or part of anotherfield, such as the opcode.

The target address field 404, 426 can be one or more fields used todetermine the target location. As examples, it can be a register fieldthat designates a register, the contents of which specify an address ofthe target location; it can be two fields, such as a base register fieldand a displacement field, in which the contents of the registerdesignated in the base register field are added to the contents of thedisplacement field to obtain an address of the target location; or anyother combination of fields, registers etc. used to designate the targetlocation.

Further, each individual branch instruction, regardless of whether it isa major or minor branch, may include one or more fields to perform theparticular branch operation. As indicated, there can be many branchinstructions, examples of which are described in the above-referencedPrinciples of Operation. For those branch instructions in the Principlesof Operation, the particular format of the branch instruction is used,but either the opcode or another field added to the format or appendedto an existing field of the instruction indicates the type of branchinstruction, e.g., minor, major Open, major Close, in accordance with anaspect of the present invention.

Additional details regarding the processing of major branch instructionsare described with reference to FIG. 5. This processing is performed bythe processor (e.g., during various pipeline stages) for each threadwithin an SMT processor (e.g., core).

Referring to FIG. 5, initially, an instruction is fetched and decoded,and an inquiry is made as to whether the instruction is a major branchOpen instruction, INQUIRY 500. If the opcode or other field of theinstruction indicates it is not a major branch Open instruction, thenprocessing continues to one or more other pipeline stages.

At another pipeline stage, a determination is made as to whether a majorbranch Close instruction is at checkpoint (i.e., complete step), INQUIRY502. If not, then processing continues to one or more other pipelinestages, and during such processing, a further inquiry is made as towhether an unexpected pipeline redirect (e.g., branch wrong, operatingsystem task swap) is being experienced, INQUIRY 504. If not, then theinstruction (or instructions) is (are) processed at each pipeline stageand then advanced to the next stage, STEP 506. Processing continues tothe next instruction.

Returning to INQUIRY 500, if it is a major branch Open instruction, thena second stream (e.g., child thread) is denoted as started/active, STEP510. In one example, this includes tracking (e.g., by a counter) thatanother thread of the pre-defined number of threads is being used.Further, a second stream (e.g., SMT thread for the child) is started atthe branch's target, STEP 512. In one example, this includes setting apointer or other indication to use the child thread, as well asstarting, in one example, instruction fetch and branch prediction forthe child thread. Since this stream or thread is a child of a parent, inthis example, the parent's architectural state is referenced by thechild. If the child changes an architectural state, then that updatedstate is associated with just the child. Processing then continues atone or more pipeline stages.

Referring to INQUIRY 502, if it is a major branch Close instruction thathas reached checkpoint, then the second stream (e.g., child thread) isallowed to checkpoint one or more instructions of its stream that havecompleted, STEP 520. The parent stream (e.g., parent SMT thread) isdeallocated, such that its resources can be used by another stream orthread, STEP 522. The child stream becomes the parent stream (e.g., bysetting a pointer or other indication that it is the parent) in whichprocessing continues where the child thread is processing. The child isconsidered complete, STEP 524. Processing then continues at one or moreother pipeline stages.

Returning to INQUIRY 504, if the parent stream (e.g., thread) encountersa branch wrong or operating system intervention, the child stream isterminated and invalidated, STEP 528. In one example, each child streamcan start another child stream via a major branch Open instruction,assuming available threads. Thus, if there are multiple child streams,then each child stream is terminated and invalidated. An indication ismade that there is no active child thread, STEP 530, and processingcontinues at STEP 506. As such, any instructions executed or to beexecuted by the child stream are now to be processed by the parentstream. Furthermore, upon encountering the associated major Closeinstruction, it is to be ignored as the close action has already takenplace. (In a further embodiment, if there are multiple child streams andone of those child streams encounters a branch wrong, then only the oneor more younger child streams are terminated.)

In a further aspect of the present invention, when employing a majorbranch Open instruction to branch to a target address, the instructionsat the target address may be treated as executing within a transaction,in which the results of the instructions are committed as one atomicunit (i.e., all committed or not). In one embodiment, the major branchOpen instruction not only starts a new stream or thread, but also startsa transaction via transactional memory. Transactional memory providesthe means to state that a given block of code is a transaction. Atransaction allows a program to checkpoint instructions at completion;however, upon the start of a transaction, the state of the machine isrecorded such that if the transaction does not complete, themachine/processor/pipeline can be restored to the point of the entry ofthe transaction. All stores are committed to memory at the end of thetransaction. This allows the state of the entire system to be able to berestored to the point at which the transaction started. In atransaction, any changes are committed together. If a transaction isabnormally terminated, the processor reverts back to a state prior tostarting the transaction. By defining the new child stream as atransaction, the child stream is allowed to perform an architecturecheckpoint in parallel with the parent stream; however, the childtransaction does not end (TEND) until the child becomes the parent.Should the parent stream encounter a branch wrong, the child stream,which is a transaction, is terminated and the state of the machine isreverted (as defined by the definition of transaction memory) as thoughno actions in the child stream took place.

In one embodiment, in order for the pipeline to determine that atransaction is to be started via transactional memory with respect todetecting a major branch Open, the architecture is modified. Asexamples, an opcode (e.g., 402, FIG. 4A) is provided that indicates abranch major Open with transaction; or format field 424, which may be apart of the opcode field or another field or separate therefrom,includes a code that indicates major branch Open with transactioninitiated.

In another embodiment, initiation of a transaction for a major branchOpen instruction is independent of the architecture and, instead, undercontrol the of hardware pipeline. Hardware control of starting atransaction with respect to major branch Open may initiate a majorbranch Open as a transaction via transactional memory when such was notspecified by the instruction. For example, if the hardware supportsnested transactions, then upon detecting a major branch Openinstruction, independent of whether the major branch Open instructionformat specifies a transaction to be started, one may be started, asdirected by the hardware. Furthermore, hardware may allow the processingof major branch Open, but deny the major branch Open from initiating asa transaction, even if such is specified by the architected format ofthe instruction. For example, if resources for processing transactionsare running low in the machine, then while the major branch Openinstruction format states to start a transaction, a transaction may beprevented from being started, so as to leave the remaining transactionalresources available for any transactions which are already in progress.Other examples also exist.

Further, with a major branch Close instruction, the following actionstake place in one embodiment: the parent stream is retired as being anactive stream; the child stream becomes the parent stream; and if themajor Branch Open started a transaction, what was the child stream is nolonger in a transactional memory state and reverts to a classicalprocessor processing state (transaction end). As such, checkpointcontent can no longer be rewound to a prior architectural checkpointstate.

One embodiment of the processing associated with a branch instructionwith transactional memory is described with reference to FIG. 6. Again,this processing is performed for each thread within an SMT processor andby the logic of the processor processing the branch instruction.

Referring to FIG. 6, a determination is made as to whether a majorbranch Open instruction is being decoded, INQUIRY 600. If the opcode orother field of the instruction indicates that a major branch Openinstruction is not being decoded, then processing continues to one ormore other pipeline stages.

At another pipe stage, a determination is made as to whether a majorbranch Close instruction is at checkpoint, INQUIRY 602. If the majorbranch Close instruction is not at checkpoint, then processing continuesto one or more other pipeline stages.

A further determination is made as to whether an unexpected pipelineredirect has occurred, INQUIRY 604. If not, the instruction (orinstructions) is (are) processed at each pipeline stage and thenadvanced to the next stage, STEP 606. Processing moves to the nextinstruction, STEP 600.

Returning to INQUIRY 600, if a major branch Open instruction is beingdecoded, then it is denoted that another stream (e.g., child thread) isbeing started/active, STEP 610, and the other stream (e.g., thread) isstarted at the branch's target, STEP 612. Further, in accordance with anaspect of the invention, a determination is made as to whether atransaction is to be started, INQUIRY 613. As one example, a field ofthe instruction (either an existing field or a new field) is used toindicate if a transaction is to be started. As another example, thehardware of the processor makes this determination either by overridingthe instruction or in lieu of the instruction having such a field thatindicates a transaction is to be started. If a transaction is to bestarted, then the new stream starts a transaction (TBEGIN), STEP 614.For the start of the transaction, a snapshot of the current state of themachine is maintained/taken such that if the transaction is abnormallyterminated, the state of the machine can be restored to the point atwhich the transaction started. Thereafter, or if a transaction is notstarted, processing continues to one or more other pipeline stages.

Returning to INQUIRY 602, if the logic determines that a major branchClose instruction is at checkpoint, then a determination is made as towhether a transaction was started, INQUIRY 616. If a transaction wasstarted, then the child stream transaction ends (TEND), STEP 618.Further, the architected state of the machine is updated to reflect thechild's transactional work, STEP 620. The parent stream (e.g., SMTthread) is deallocated and is no longer active, STEP 622. The childstream is made to be the parent stream and it is noted that the child isno longer active, STEP 624. Processing then continues in the pipeline.

Referring to INQUIRY 616, if a transaction is not started, then anycomplete child instructions are allowed to checkpoint, STEP 621, andprocessing continues to STEP 622.

Returning to INQUIRY 604, if an unexpected pipeline redirect has beenencountered, then a determination is made as to whether a transactionwas started, INQUIRY 626. If a transaction was started, then the childstream's transaction is abnormally terminated (TABORT), STEP 630.Thereafter, or if a transaction was not started, the child stream isdeallocated and invalidated, STEP 632; and it is noted that the child isno longer active, STEP 634. In one embodiment, more than one childstream can be in process. If the redirect is encountered on the parent,then all the child streams having any association with the parent areended, including abnormally terminating the transaction. If the redirectis at a child stream, then any younger child streams are to be ended,including abnormally terminating the transaction. Further, if a childstream encounters an interrupt or exception, the child stream pausesprocess and resumes process upon becoming the parent stream. Processingcontinues to STEP 606.

The starting of the child stream(s) in transactional memory allows thechild stream(s) to run further ahead. A child stream no longer has towithhold pipelining resources to prevent a hang-up in the parent thread.Further, the child stream is able to retire instructions from theout-of-order completion tables which were limited in size, typicallysupporting around 200 instructions maximum. In one embodiment, childstreams still maintain some limits on the amount of completion that canbe performed as a transaction, for resources are required to maintainall of the store data which is to be committed upon completion of thetransaction. The hardware to maintain the stores until transactioncompletion is a lot less restrictive than those resources for theout-of-order windows with respect to, for instance, frequency, latency,capacity, and/or floor plan placement.

Described above is a branch mechanism that enables one segment of codeof a program to be processed concurrent to at least one other segment ofcode of the same program. In one example, a branch instruction isprovided that starts a parallel thread (e.g., a child thread) in thetaken path, but keeps executing (e.g., parent thread) in the non-takenpath. The parent thread includes instructions for either ending thethread or joining the other thread. In one embodiment, the child threaddoes not update architected state (i.e., state visible to otherapplications, threads, etc.) until the parent thread reaches a definedpoint in processing, such as a major branch Close instruction. Further,in one embodiment, the thread of the taken path starts a transactionalmemory transaction.

The use of major branches allows instructions to be decoded/dispatchedout-of-order, so as to provide a larger out-of-order window forinstructions to be executed. Without a major branch OPEN/CLOSE,instructions are decoded and dispatched in-order, but may be issuedout-of-order, and are completed in-order.

In one embodiment, the amount of processing of the child streams islimited to the out-of-order resources available in the processor. Thechild streams while using out-of-order resources are not to consume allresources as to create a hang situation such that the parent is unableto obtain resources it needs to make forward progress. This limitation,however, is avoided if the child stream uses transactional processing.

In one embodiment, if a major branch OPEN is encountered and all threadsare in use for spanning off child paths, then the major branch OPEN isnot executed. This is denoted and upon encountering the paired majorbranch CLOSE, it is also not executed. By not executing on the CLOSE, aparent to child thread switch will not take place.

In one example, a restart of the parent stream, including a restart ofthe I-fetch fetching stream, terminates the child stream of processingand any of its children, etc. A restart in a child stream terminates itschild and any younger children, but not the parent.

The hardware (e.g., transistors) that define a processor pipeline ismade up of many queues. The queues are finite in size, and such queuesinclude, but are not limited to: cache line addresses, instructions todecode (post instruction fetch), instructions to dispatch, instructionsfrom dispatch to architectural checkpoint, general purpose registers,branch queues, and store queues. Without transactional processing, aprocessor may require a subset or all of these queues to empty in anin-order fashion, even in an out-of-order processor. Those resources(e.g., queues) claimed by a child stream may not be emptied until theparent stream has completed. If the child stream occupies too manyresources, then the parent may not have enough resources available toperform the operations of instructions as defined by the architectureand ordered by the software program. It is to prevent such a hangsituation that the child may not use resources to the point that thereare not enough resources available to the parent for the parent to makeforward progress. For the child to be able to limit its resource usageit is to know how much resource is available for it to use. This amountof resource may be either hardwired in the hardware or the parent threadcan communicate to the child how much resource is available to be used.Furthermore, the parent may communicate an under-amount of resourcesthat are available for the child to use such that, while the child doesnot hang the parent, the child does not use so much resource as to bringthe parent's rate of forward progress to a crawl.

In a further embodiment, branch prediction is allowed to proceed down achild stream in the manner it would proceed down two independent threadsin, for instance, an SMT-2 environment. Branch prediction may be used topredict whether a branch is to be taken and/or the target location ofthat branch.

As will be appreciated by one skilled in the art, one or more aspects ofthe present invention may be embodied as a system, method or computerprogram product. Accordingly, one or more aspects of the presentinvention may take the form of an entirely hardware embodiment, anentirely software embodiment (including firmware, resident software,micro-code, etc.) or an embodiment combining software and hardwareaspects that may all generally be referred to herein as a “circuit,”“module” or “system”. Furthermore, one or more aspects of the presentinvention may take the form of a computer program product embodied inone or more computer readable medium(s) having computer readable programcode embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readablestorage medium. A computer readable storage medium may be, for example,but not limited to, an electronic, magnetic, optical, electromagnetic,infrared or semiconductor system, apparatus, or device, or any suitablecombination of the foregoing. More specific examples (a non-exhaustivelist) of the computer readable storage medium include the following: anelectrical connection having one or more wires, a portable computerdiskette, a hard disk, a random access memory (RAM), a read-only memory(ROM), an erasable programmable read-only memory (EPROM or Flashmemory), an optical fiber, a portable compact disc read-only memory(CD-ROM), an optical storage device, a magnetic storage device, or anysuitable combination of the foregoing. In the context of this document,a computer readable storage medium may be any tangible medium that cancontain or store a program for use by or in connection with aninstruction execution system, apparatus, or device.

Referring now to FIG. 7, in one example, a computer program product 700includes, for instance, one or more non-transitory computer readablestorage media 702 to store computer readable program code means or logic704 thereon to provide and facilitate one or more aspects of the presentinvention.

Program code embodied on a computer readable medium may be transmittedusing an appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for one or moreaspects of the present invention may be written in any combination ofone or more programming languages, including an object orientedprogramming language, such as Java, Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language, assembler or similar programming languages. Theprogram code may execute entirely on the user's computer, partly on theuser's computer, as a stand-alone software package, partly on the user'scomputer and partly on a remote computer or entirely on the remotecomputer or server. In the latter scenario, the remote computer may beconnected to the user's computer through any type of network, includinga local area network (LAN) or a wide area network (WAN), or theconnection may be made to an external computer (for example, through theInternet using an Internet Service Provider).

One or more aspects of the present invention are described herein withreference to flowchart illustrations and/or block diagrams of methods,apparatus (systems) and computer program products according toembodiments of the invention. It will be understood that each block ofthe flowchart illustrations and/or block diagrams, and combinations ofblocks in the flowchart illustrations and/or block diagrams, can beimplemented by computer program instructions. These computer programinstructions may be provided to a processor of a general purposecomputer, special purpose computer, or other programmable dataprocessing apparatus to produce a machine, such that the instructions,which execute via the processor of the computer or other programmabledata processing apparatus, create means for implementing thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of one or more aspects of the present invention. In thisregard, each block in the flowchart or block diagrams may represent amodule, segment, or portion of code, which comprises one or moreexecutable instructions for implementing the specified logicalfunction(s). It should also be noted that, in some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

In addition to the above, one or more aspects of the present inventionmay be provided, offered, deployed, managed, serviced, etc. by a serviceprovider who offers management of customer environments. For instance,the service provider can create, maintain, support, etc. computer codeand/or a computer infrastructure that performs one or more aspects ofthe present invention for one or more customers. In return, the serviceprovider may receive payment from the customer under a subscriptionand/or fee agreement, as examples. Additionally or alternatively, theservice provider may receive payment from the sale of advertisingcontent to one or more third parties.

In one aspect of the present invention, an application may be deployedfor performing one or more aspects of the present invention. As oneexample, the deploying of an application comprises providing computerinfrastructure operable to perform one or more aspects of the presentinvention.

As a further aspect of the present invention, a computing infrastructuremay be deployed comprising integrating computer readable code into acomputing system, in which the code in combination with the computingsystem is capable of performing one or more aspects of the presentinvention.

As yet a further aspect of the present invention, a process forintegrating computing infrastructure comprising integrating computerreadable code into a computer system may be provided. The computersystem comprises a computer readable medium, in which the computermedium comprises one or more aspects of the present invention. The codein combination with the computer system is capable of performing one ormore aspects of the present invention.

Although various embodiments are described above, these are onlyexamples. For example, other computing environments can incorporate anduse one or more aspects of the present invention. For instance, acomputing environment may include a central processor complex having aplurality of processors. The central processor complex may or may not belogically partitioned. Further, computing environments of otherarchitectures can incorporate and use one or more aspects of the presentinvention. Additionally, while major branch Open and Close aredescribed, they may be called by different names or additional majorbranch types may be provided. Many changes and/or additions may be madewithout departing from the spirit of the present invention.

Further, other types of computing environments can benefit from one ormore aspects of the present invention. As an example, a data processingsystem suitable for storing and/or executing program code is usable thatincludes at least two processors coupled directly or indirectly tomemory elements through a system bus. The memory elements include, forinstance, local memory employed during actual execution of the programcode, bulk storage, and cache memory which provide temporary storage ofat least some program code in order to reduce the number of times codemust be retrieved from bulk storage during execution.

Input/Output or I/O devices (including, but not limited to, keyboards,displays, pointing devices, DASD, tape, CDs, DVDs, thumb drives andother memory media, etc.) can be coupled to the system either directlyor through intervening I/O controllers. Network adapters may also becoupled to the system to enable the data processing system to becomecoupled to other data processing systems or remote printers or storagedevices through intervening private or public networks. Modems, cablemodems, and Ethernet cards are just a few of the available types ofnetwork adapters.

Other examples of computing environments that may incorporate and/or useone or more aspects of the present invention are described below.

Referring to FIG. 8, representative components of a Host Computer system5000 to implement one or more aspects of the present invention areportrayed. The representative host computer 5000 comprises one or moreCPUs 5001 in communication with computer memory (i.e., central storage)5002, as well as I/O interfaces to storage media devices 5011 andnetworks 5010 for communicating with other computers or SANs and thelike. The CPU 5001 is compliant with an architecture having anarchitected instruction set and architected functionality. The CPU 5001may have dynamic address translation (DAT) 5003 for transforming programaddresses (virtual addresses) into real addresses of memory. A DATtypically includes a translation lookaside buffer (TLB) 5007 for cachingtranslations so that later accesses to the block of computer memory 5002do not require the delay of address translation. Typically, a cache 5009is employed between computer memory 5002 and the processor 5001. Thecache 5009 may be hierarchical having a large cache available to morethan one CPU and smaller, faster (lower level) caches between the largecache and each CPU. In some implementations, the lower level caches aresplit to provide separate low level caches for instruction fetching anddata accesses. In one embodiment, an instruction is fetched from memory5002 by an instruction fetch unit 5004 via a cache 5009. The instructionis decoded in an instruction decode unit 5006 and dispatched (with otherinstructions in some embodiments) to instruction execution unit or units5008. Typically several execution units 5008 are employed, for examplean arithmetic execution unit, a floating point execution unit and abranch instruction execution unit. The instruction is executed by theexecution unit, accessing operands from instruction specified registersor memory as needed. If an operand is to be accessed (loaded or stored)from memory 5002, a load/store unit 5005 typically handles the accessunder control of the instruction being executed. Instructions may beexecuted in hardware circuits or in internal microcode (firmware) or bya combination of both.

As noted, a computer system includes information in local (or main)storage, as well as addressing, protection, and reference and changerecording. Some aspects of addressing include the format of addresses,the concept of address spaces, the various types of addresses, and themanner in which one type of address is translated to another type ofaddress. Some of main storage includes permanently assigned storagelocations. Main storage provides the system with directly addressablefast-access storage of data. Both data and programs are to be loadedinto main storage (from input devices) before they can be processed.

Main storage may include one or more smaller, faster-access bufferstorages, sometimes called caches. A cache is typically physicallyassociated with a CPU or an I/O processor. The effects, except onperformance, of the physical construction and use of distinct storagemedia are generally not observable by the program.

Separate caches may be maintained for instructions and for dataoperands. Information within a cache is maintained in contiguous byteson an integral boundary called a cache block or cache line (or line, forshort). A model may provide an EXTRACT CACHE ATTRIBUTE instruction whichreturns the size of a cache line in bytes. A model may also providePREFETCH DATA and PREFETCH DATA RELATIVE LONG instructions which effectsthe prefetching of storage into the data or instruction cache or thereleasing of data from the cache.

Storage is viewed as a long horizontal string of bits. For mostoperations, accesses to storage proceed in a left-to-right sequence. Thestring of bits is subdivided into units of eight bits. An eight-bit unitis called a byte, which is the basic building block of all informationformats. Each byte location in storage is identified by a uniquenonnegative integer, which is the address of that byte location or,simply, the byte address. Adjacent byte locations have consecutiveaddresses, starting with 0 on the left and proceeding in a left-to-rightsequence. Addresses are unsigned binary integers and are 24, 31, or 64bits.

Information is transmitted between storage and a CPU or a channelsubsystem one byte, or a group of bytes, at a time. Unless otherwisespecified, in, for instance, the z/Architecture®, a group of bytes instorage is addressed by the leftmost byte of the group. The number ofbytes in the group is either implied or explicitly specified by theoperation to be performed. When used in a CPU operation, a group ofbytes is called a field. Within each group of bytes, in, for instance,the z/Architecture®, bits are numbered in a left-to-right sequence. Inthe z/Architecture®, the leftmost bits are sometimes referred to as the“high-order” bits and the rightmost bits as the “low-order” bits. Bitnumbers are not storage addresses, however. Only bytes can be addressed.To operate on individual bits of a byte in storage, the entire byte isaccessed. The bits in a byte are numbered 0 through 7, from left toright (in, e.g., the z/Architecture). The bits in an address may benumbered 8-31 or 40-63 for 24-bit addresses, or 1-31 or 33-63 for 31-bitaddresses; they are numbered 0-63 for 64-bit addresses. Within any otherfixed-length format of multiple bytes, the bits making up the format areconsecutively numbered starting from 0. For purposes of error detection,and in preferably for correction, one or more check bits may betransmitted with each byte or with a group of bytes. Such check bits aregenerated automatically by the machine and cannot be directly controlledby the program. Storage capacities are expressed in number of bytes.When the length of a storage-operand field is implied by the operationcode of an instruction, the field is said to have a fixed length, whichcan be one, two, four, eight, or sixteen bytes. Larger fields may beimplied for some instructions. When the length of a storage-operandfield is not implied but is stated explicitly, the field is said to havea variable length. Variable-length operands can vary in length byincrements of one byte (or with some instructions, in multiples of twobytes or other multiples). When information is placed in storage, thecontents of only those byte locations are replaced that are included inthe designated field, even though the width of the physical path tostorage may be greater than the length of the field being stored.

Certain units of information are to be on an integral boundary instorage. A boundary is called integral for a unit of information whenits storage address is a multiple of the length of the unit in bytes.Special names are given to fields of 2, 4, 8, and 16 bytes on anintegral boundary. A halfword is a group of two consecutive bytes on atwo-byte boundary and is the basic building block of instructions. Aword is a group of four consecutive bytes on a four-byte boundary. Adoubleword is a group of eight consecutive bytes on an eight-byteboundary. A quadword is a group of 16 consecutive bytes on a 16-byteboundary. When storage addresses designate halfwords, words,doublewords, and quadwords, the binary representation of the addresscontains one, two, three, or four rightmost zero bits, respectively.Instructions are to be on two-byte integral boundaries. The storageoperands of most instructions do not have boundary-alignmentrequirements.

On devices that implement separate caches for instructions and dataoperands, a significant delay may be experienced if the program storesinto a cache line from which instructions are subsequently fetched,regardless of whether the store alters the instructions that aresubsequently fetched.

In one embodiment, the invention may be practiced by software (sometimesreferred to licensed internal code, firmware, micro-code, milli-code,pico-code and the like, any of which would be consistent with one ormore aspects the present invention). Referring to FIG. 8, softwareprogram code which embodies one or more aspects of the present inventionmay be accessed by processor 5001 of the host system 5000 from long-termstorage media devices 5011, such as a CD-ROM drive, tape drive or harddrive. The software program code may be embodied on any of a variety ofknown media for use with a data processing system, such as a diskette,hard drive, or CD-ROM. The code may be distributed on such media, or maybe distributed to users from computer memory 5002 or storage of onecomputer system over a network 5010 to other computer systems for use byusers of such other systems.

The software program code includes an operating system which controlsthe function and interaction of the various computer components and oneor more application programs. Program code is normally paged fromstorage media device 5011 to the relatively higher-speed computerstorage 5002 where it is available for processing by processor 5001. Thetechniques and methods for embodying software program code in memory, onphysical media, and/or distributing software code via networks are wellknown and will not be further discussed herein. Program code, whencreated and stored on a tangible medium (including but not limited toelectronic memory modules (RAM), flash memory, Compact Discs (CDs),DVDs, Magnetic Tape and the like is often referred to as a “computerprogram product”. The computer program product medium is typicallyreadable by a processing circuit preferably in a computer system forexecution by the processing circuit.

FIG. 9 illustrates a representative workstation or server hardwaresystem in which one or more aspects of the present invention may bepracticed. The system 5020 of FIG. 9 comprises a representative basecomputer system 5021, such as a personal computer, a workstation or aserver, including optional peripheral devices. The base computer system5021 includes one or more processors 5026 and a bus employed to connectand enable communication between the processor(s) 5026 and the othercomponents of the system 5021 in accordance with known techniques. Thebus connects the processor 5026 to memory 5025 and long-term storage5027 which can include a hard drive (including any of magnetic media,CD, DVD and Flash Memory for example) or a tape drive for example. Thesystem 5021 might also include a user interface adapter, which connectsthe microprocessor 5026 via the bus to one or more interface devices,such as a keyboard 5024, a mouse 5023, a printer/scanner 5030 and/orother interface devices, which can be any user interface device, such asa touch sensitive screen, digitized entry pad, etc. The bus alsoconnects a display device 5022, such as an LCD screen or monitor, to themicroprocessor 5026 via a display adapter.

The system 5021 may communicate with other computers or networks ofcomputers by way of a network adapter capable of communicating 5028 witha network 5029. Example network adapters are communications channels,token ring, Ethernet or modems. Alternatively, the system 5021 maycommunicate using a wireless interface, such as a CDPD (cellular digitalpacket data) card. The system 5021 may be associated with such othercomputers in a Local Area Network (LAN) or a Wide Area Network (WAN), orthe system 5021 can be a client in a client/server arrangement withanother computer, etc. All of these configurations, as well as theappropriate communications hardware and software, are known in the art.

FIG. 10 illustrates a data processing network 5040 in which one or moreaspects of the present invention may be practiced. The data processingnetwork 5040 may include a plurality of individual networks, such as awireless network and a wired network, each of which may include aplurality of individual workstations 5041, 5042, 5043, 5044.Additionally, as those skilled in the art will appreciate, one or moreLANs may be included, where a LAN may comprise a plurality ofintelligent workstations coupled to a host processor.

Still referring to FIG. 10, the networks may also include mainframecomputers or servers, such as a gateway computer (client server 5046) orapplication server (remote server 5048 which may access a datarepository and may also be accessed directly from a workstation 5045). Agateway computer 5046 serves as a point of entry into each individualnetwork. A gateway is needed when connecting one networking protocol toanother. The gateway 5046 may be preferably coupled to another network(the Internet 5047 for example) by means of a communications link. Thegateway 5046 may also be directly coupled to one or more workstations5041, 5042, 5043, 5044 using a communications link. The gateway computermay be implemented utilizing an IBM eServer™ System z® server availablefrom International Business Machines Corporation.

Referring concurrently to FIG. 9 and FIG. 10, software programming codewhich may embody one or more aspects of the present invention may beaccessed by the processor 5026 of the system 5020 from long-term storagemedia 5027, such as a CD-ROM drive or hard drive. The softwareprogramming code may be embodied on any of a variety of known media foruse with a data processing system, such as a diskette, hard drive, orCD-ROM. The code may be distributed on such media, or may be distributedto users 5050, 5051 from the memory or storage of one computer systemover a network to other computer systems for use by users of such othersystems.

Alternatively, the programming code may be embodied in the memory 5025,and accessed by the processor 5026 using the processor bus. Suchprogramming code includes an operating system which controls thefunction and interaction of the various computer components and one ormore application programs 5032. Program code is normally paged fromstorage media 5027 to high-speed memory 5025 where it is available forprocessing by the processor 5026. The techniques and methods forembodying software programming code in memory, on physical media, and/ordistributing software code via networks are well known and will not befurther discussed herein. Program code, when created and stored on atangible medium (including but not limited to electronic memory modules(RAM), flash memory, Compact Discs (CDs), DVDs, Magnetic Tape and thelike is often referred to as a “computer program product”. The computerprogram product medium is typically readable by a processing circuitpreferably in a computer system for execution by the processing circuit.

The cache that is most readily available to the processor (normallyfaster and smaller than other caches of the processor) is the lowest (L1or level one) cache and main store (main memory) is the highest levelcache (L3 if there are 3 levels). The lowest level cache is oftendivided into an instruction cache (I-Cache) holding machine instructionsto be executed and a data cache (D-Cache) holding data operands.

Referring to FIG. 11, an exemplary processor embodiment is depicted forprocessor 5026. Typically one or more levels of cache 5053 are employedto buffer memory blocks in order to improve processor performance. Thecache 5053 is a high speed buffer holding cache lines of memory datathat are likely to be used. Typical cache lines are 64, 128 or 256 bytesof memory data. Separate caches are often employed for cachinginstructions than for caching data. Cache coherence (synchronization ofcopies of lines in memory and the caches) is often provided by various“snoop” algorithms well known in the art. Main memory storage 5025 of aprocessor system is often referred to as a cache. In a processor systemhaving 4 levels of cache 5053, main storage 5025 is sometimes referredto as the level 5 (L5) cache since it is typically faster and only holdsa portion of the non-volatile storage (DASD, tape etc) that is availableto a computer system. Main storage 5025 “caches” pages of data paged inand out of the main storage 5025 by the operating system.

A program counter (instruction counter) 5061 keeps track of the addressof the current instruction to be executed. A program counter in az/Architecture® processor is 64 bits and can be truncated to 31 or 24bits to support prior addressing limits. A program counter is typicallyembodied in a PSW (program status word) of a computer such that itpersists during context switching. Thus, a program in progress, having aprogram counter value, may be interrupted by, for example, the operatingsystem (context switch from the program environment to the operatingsystem environment). The PSW of the program maintains the programcounter value while the program is not active, and the program counter(in the PSW) of the operating system is used while the operating systemis executing. Typically, the program counter is incremented by an amountequal to the number of bytes of the current instruction. RISC (ReducedInstruction Set Computing) instructions are typically fixed length whileCISC (Complex Instruction Set Computing) instructions are typicallyvariable length. Instructions of the IBM z/Architecture® are CISCinstructions having a length of 2, 4 or 6 bytes. The Program counter5061 is modified by either a context switch operation or a branch takenoperation of a branch instruction for example. In a context switchoperation, the current program counter value is saved in the programstatus word along with other state information about the program beingexecuted (such as condition codes), and a new program counter value isloaded pointing to an instruction of a new program module to beexecuted. A branch taken operation is performed in order to permit theprogram to make decisions or loop within the program by loading theresult of the branch instruction into the program counter 5061.

Typically an instruction fetch unit 5055 is employed to fetchinstructions on behalf of the processor 5026. The fetch unit eitherfetches “next sequential instructions”, target instructions of branchtaken instructions, or first instructions of a program following acontext switch. Modern Instruction fetch units often employ prefetchtechniques to speculatively prefetch instructions based on thelikelihood that the prefetched instructions might be used. For example,a fetch unit may fetch 16 bytes of instruction that includes the nextsequential instruction and additional bytes of further sequentialinstructions.

The fetched instructions are then executed by the processor 5026. In anembodiment, the fetched instruction(s) are passed to a dispatch unit5056 of the fetch unit. The dispatch unit decodes the instruction(s) andforwards information about the decoded instruction(s) to appropriateunits 5057, 5058, 5060. An execution unit 5057 will typically receiveinformation about decoded arithmetic instructions from the instructionfetch unit 5055 and will perform arithmetic operations on operandsaccording to the opcode of the instruction. Operands are provided to theexecution unit 5057 preferably either from memory 5025, architectedregisters 5059 or from an immediate field of the instruction beingexecuted. Results of the execution, when stored, are stored either inmemory 5025, registers 5059 or in other machine hardware (such ascontrol registers, PSW registers and the like).

A processor 5026 typically has one or more units 5057, 5058, 5060 forexecuting the function of the instruction. Referring to FIG. 12A, anexecution unit 5057 may communicate with architected general registers5059, a decode/dispatch unit 5056, a load store unit 5060, and other5065 processor units by way of interfacing logic 5071. An execution unit5057 may employ several register circuits 5067, 5068, 5069 to holdinformation that the arithmetic logic unit (ALU) 5066 will operate on.The ALU performs arithmetic operations such as add, subtract, multiplyand divide as well as logical function such as and, or and exclusive-or(XOR), rotate and shift. Preferably the ALU supports specializedoperations that are design dependent. Other circuits may provide otherarchitected facilities 5072 including condition codes and recoverysupport logic for example. Typically the result of an ALU operation isheld in an output register circuit 5070 which can forward the result toa variety of other processing functions. There are many arrangements ofprocessor units, the present description is only intended to provide arepresentative understanding of one embodiment.

An ADD instruction for example would be executed in an execution unit5057 having arithmetic and logical functionality while a floating pointinstruction for example would be executed in a floating point executionhaving specialized floating point capability. Preferably, an executionunit operates on operands identified by an instruction by performing anopcode defined function on the operands. For example, an ADD instructionmay be executed by an execution unit 5057 on operands found in tworegisters 5059 identified by register fields of the instruction.

The execution unit 5057 performs the arithmetic addition on two operandsand stores the result in a third operand where the third operand may bea third register or one of the two source registers. The execution unitpreferably utilizes an Arithmetic Logic Unit (ALU) 5066 that is capableof performing a variety of logical functions such as Shift, Rotate, And,Or and XOR as well as a variety of algebraic functions including any ofadd, subtract, multiply, divide. Some ALUs 5066 are designed for scalaroperations and some for floating point. Data may be Big Endian (wherethe least significant byte is at the highest byte address) or LittleEndian (where the least significant byte is at the lowest byte address)depending on architecture. The IBM z/Architecture® is Big Endian. Signedfields may be sign and magnitude, 1's complement or 2's complementdepending on architecture. A 2's complement number is advantageous inthat the ALU does not need to design a subtract capability since eithera negative value or a positive value in 2's complement requires only anaddition within the ALU. Numbers are commonly described in shorthand,where a 12 bit field defines an address of a 4,096 byte block and iscommonly described as a 4 Kbyte (Kilo-byte) block, for example.

Referring to FIG. 12B, branch instruction information for executing abranch instruction is typically sent to a branch unit 5058 which oftenemploys a branch prediction algorithm such as a branch history table5082 to predict the outcome of the branch before other conditionaloperations are complete. The target of the current branch instructionwill be fetched and speculatively executed before the conditionaloperations are complete. When the conditional operations are completedthe speculatively executed branch instructions are either completed ordiscarded based on the conditions of the conditional operation and thespeculated outcome. A typical branch instruction may test conditioncodes and branch to a target address if the condition codes meet thebranch requirement of the branch instruction, a target address may becalculated based on several numbers including ones found in registerfields or an immediate field of the instruction for example. The branchunit 5058 may employ an ALU 5074 having a plurality of input registercircuits 5075, 5076, 5077 and an output register circuit 5080. Thebranch unit 5058 may communicate with general registers 5059, decodedispatch unit 5056 or other circuits 5073, for example.

The execution of a group of instructions can be interrupted for avariety of reasons including a context switch initiated by an operatingsystem, a program exception or error causing a context switch, an I/Ointerruption signal causing a context switch or multi-threading activityof a plurality of programs (in a multi-threaded environment), forexample. Preferably a context switch action saves state informationabout a currently executing program and then loads state informationabout another program being invoked. State information may be saved inhardware registers or in memory for example. State informationpreferably comprises a program counter value pointing to a nextinstruction to be executed, condition codes, memory translationinformation and architected register content. A context switch activitycan be exercised by hardware circuits, application programs, operatingsystem programs or firmware code (microcode, pico-code or licensedinternal code (LIC)) alone or in combination.

A processor accesses operands according to instruction defined methods.The instruction may provide an immediate operand using the value of aportion of the instruction, may provide one or more register fieldsexplicitly pointing to either general purpose registers or specialpurpose registers (floating point registers for example). Theinstruction may utilize implied registers identified by an opcode fieldas operands. The instruction may utilize memory locations for operands.A memory location of an operand may be provided by a register, animmediate field, or a combination of registers and immediate field asexemplified by the z/Architecture® long displacement facility whereinthe instruction defines a base register, an index register and animmediate field (displacement field) that are added together to providethe address of the operand in memory for example. Location hereintypically implies a location in main memory (main storage) unlessotherwise indicated.

Referring to FIG. 12C, a processor accesses storage using a load/storeunit 5060. The load/store unit 5060 may perform a load operation byobtaining the address of the target operand in memory 5053 and loadingthe operand in a register 5059 or another memory 5053 location, or mayperform a store operation by obtaining the address of the target operandin memory 5053 and storing data obtained from a register 5059 or anothermemory 5053 location in the target operand location in memory 5053. Theload/store unit 5060 may be speculative and may access memory in asequence that is out-of-order relative to instruction sequence, howeverthe load/store unit 5060 is to maintain the appearance to programs thatinstructions were executed in order. A load/store unit 5060 maycommunicate with general registers 5059, decode/dispatch unit 5056,cache/memory interface 5053 or other elements 5083 and comprises variousregister circuits, ALUs 5085 and control logic 5090 to calculate storageaddresses and to provide pipeline sequencing to keep operationsin-order. Some operations may be out of order but the load/store unitprovides functionality to make the out of order operations to appear tothe program as having been performed in order, as is well known in theart.

Preferably addresses that an application program “sees” are oftenreferred to as virtual addresses. Virtual addresses are sometimesreferred to as “logical addresses” “effective addresses”. These virtualaddresses are virtual in that they are redirected to physical memorylocation by one of a variety of dynamic address translation (DAT)technologies including, but not limited to, simply prefixing a virtualaddress with an offset value, translating the virtual address via one ormore translation tables, the translation tables preferably comprising atleast a segment table and a page table alone or in combination,preferably, the segment table having an entry pointing to the pagetable. In the z/Architecture®, a hierarchy of translation is providedincluding a region first table, a region second table, a region thirdtable, a segment table and an optional page table. The performance ofthe address translation is often improved by utilizing a translationlookaside buffer (TLB) which comprises entries mapping a virtual addressto an associated physical memory location. The entries are created whenthe DAT translates a virtual address using the translation tables.Subsequent use of the virtual address can then utilize the entry of thefast TLB rather than the slow sequential translation table accesses. TLBcontent may be managed by a variety of replacement algorithms includingLRU (Least Recently used).

In the case where the processor is a processor of a multi-processorsystem, each processor has responsibility to keep shared resources, suchas I/O, caches, TLBs and memory, interlocked for coherency. Typically,“snoop” technologies will be utilized in maintaining cache coherency. Ina snoop environment, each cache line may be marked as being in any oneof a shared state, an exclusive state, a changed state, an invalid stateand the like in order to facilitate sharing.

I/O units 5054 (FIG. 11) provide the processor with means for attachingto peripheral devices including tape, disc, printers, displays, andnetworks for example. I/O units are often presented to the computerprogram by software drivers. In mainframes, such as the System z® fromIBM®, channel adapters and open system adapters are I/O units of themainframe that provide the communications between the operating systemand peripheral devices.

Further, other types of computing environments can benefit from one ormore aspects of the present invention. As an example, as mentionedherein, an environment may include an emulator (e.g., software or otheremulation mechanisms), in which a particular architecture (including,for instance, instruction execution, architected functions, such asaddress translation, and architected registers) or a subset thereof isemulated (e.g., on a native computer system having a processor andmemory). In such an environment, one or more emulation functions of theemulator can implement one or more aspects of the present invention,even though a computer executing the emulator may have a differentarchitecture than the capabilities being emulated. As one example, inemulation mode, the specific instruction or operation being emulated isdecoded, and an appropriate emulation function is built to implement theindividual instruction or operation.

In an emulation environment, a host computer includes, for instance, amemory to store instructions and data; an instruction fetch unit tofetch instructions from memory and to optionally, provide localbuffering for the fetched instruction; an instruction decode unit toreceive the fetched instructions and to determine the type ofinstructions that have been fetched; and an instruction execution unitto execute the instructions. Execution may include loading data into aregister from memory; storing data back to memory from a register; orperforming some type of arithmetic or logical operation, as determinedby the decode unit. In one example, each unit is implemented insoftware. For instance, the operations being performed by the units areimplemented as one or more subroutines within emulator software.

More particularly, in a mainframe, architected machine instructions areused by programmers, usually today “C” programmers, often by way of acompiler application. These instructions stored in the storage mediummay be executed natively in a z/Architecture® IBM® Server, oralternatively in machines executing other architectures. They can beemulated in the existing and in future IBM® mainframe servers and onother machines of IBM® (e.g., Power Systems servers and System x®Servers). They can be executed in machines running Linux on a widevariety of machines using hardware manufactured by IBM®, Intel®, AMD™,and others. Besides execution on that hardware under a z/Architecture®,Linux can be used as well as machines which use emulation by Hercules,UMX, or FSI (Fundamental Software, Inc), where generally execution is inan emulation mode. In emulation mode, emulation software is executed bya native processor to emulate the architecture of an emulated processor.

The native processor typically executes emulation software comprisingeither firmware or a native operating system to perform emulation of theemulated processor. The emulation software is responsible for fetchingand executing instructions of the emulated processor architecture. Theemulation software maintains an emulated program counter to keep trackof instruction boundaries. The emulation software may fetch one or moreemulated machine instructions at a time and convert the one or moreemulated machine instructions to a corresponding group of native machineinstructions for execution by the native processor. These convertedinstructions may be cached such that a faster conversion can beaccomplished. Notwithstanding, the emulation software is to maintain thearchitecture rules of the emulated processor architecture so as toassure operating systems and applications written for the emulatedprocessor operate correctly. Furthermore, the emulation software is toprovide resources identified by the emulated processor architectureincluding, but not limited to, control registers, general purposeregisters, floating point registers, dynamic address translationfunction including segment tables and page tables for example, interruptmechanisms, context switch mechanisms, Time of Day (TOD) clocks andarchitected interfaces to I/O subsystems such that an operating systemor an application program designed to run on the emulated processor, canbe run on the native processor having the emulation software.

A specific instruction being emulated is decoded, and a subroutine iscalled to perform the function of the individual instruction. Anemulation software function emulating a function of an emulatedprocessor is implemented, for example, in a “C” subroutine or driver, orsome other method of providing a driver for the specific hardware aswill be within the skill of those in the art after understanding thedescription of the preferred embodiment. Various software and hardwareemulation patents including, but not limited to U.S. Pat. No. 5,551,013,entitled “Multiprocessor for Hardware Emulation”, by Beausoleil et al.;and U.S. Pat. No. 6,009,261, entitled “Preprocessing of Stored TargetRoutines for Emulating Incompatible Instructions on a Target Processor”,by Scalzi et al; and U.S. Pat. No. 5,574,873, entitled “Decoding GuestInstruction to Directly Access Emulation Routines that Emulate the GuestInstructions”, by Davidian et al; and U.S. Pat. No. 6,308,255, entitled“Symmetrical Multiprocessing Bus and Chipset Used for CoprocessorSupport Allowing Non-Native Code to Run in a System”, by Gorishek et al;and U.S. Pat. No. 6,463,582, entitled “Dynamic Optimizing Object CodeTranslator for Architecture Emulation and Dynamic Optimizing Object CodeTranslation Method”, by Lethin et al; and U.S. Pat. No. 5,790,825,entitled “Method for Emulating Guest Instructions on a Host ComputerThrough Dynamic Recompilation of Host Instructions”, by Eric Traut, eachof which is hereby incorporated herein by reference in its entirety; andmany others, illustrate a variety of known ways to achieve emulation ofan instruction format architected for a different machine for a targetmachine available to those skilled in the art.

In FIG. 13, an example of an emulated host computer system 5092 isprovided that emulates a host computer system 5000′ of a hostarchitecture. In the emulated host computer system 5092, the hostprocessor (CPU) 5091 is an emulated host processor (or virtual hostprocessor) and comprises an emulation processor 5093 having a differentnative instruction set architecture than that of the processor 5091 ofthe host computer 5000′. The emulated host computer system 5092 hasmemory 5094 accessible to the emulation processor 5093. In the exampleembodiment, the memory 5094 is partitioned into a host computer memory5096 portion and an emulation routines 5097 portion. The host computermemory 5096 is available to programs of the emulated host computer 5092according to host computer architecture. The emulation processor 5093executes native instructions of an architected instruction set of anarchitecture other than that of the emulated processor 5091, the nativeinstructions obtained from emulation routines memory 5097, and mayaccess a host instruction for execution from a program in host computermemory 5096 by employing one or more instruction(s) obtained in asequence & access/decode routine which may decode the hostinstruction(s) accessed to determine a native instruction executionroutine for emulating the function of the host instruction accessed.Other facilities that are defined for the host computer system 5000′architecture may be emulated by architected facilities routines,including such facilities as general purpose registers, controlregisters, dynamic address translation and I/O subsystem support andprocessor cache, for example. The emulation routines may also takeadvantage of functions available in the emulation processor 5093 (suchas general registers and dynamic translation of virtual addresses) toimprove performance of the emulation routines. Special hardware andoff-load engines may also be provided to assist the processor 5093 inemulating the function of the host computer 5000′.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising”, when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below, if any, areintended to include any structure, material, or act for performing thefunction in combination with other claimed elements as specificallyclaimed. The description of one or more aspects of the present inventionhas been presented for purposes of illustration and description, but isnot intended to be exhaustive or limited to the invention in the formdisclosed. Many modifications and variations will be apparent to thoseof ordinary skill in the art without departing from the scope and spiritof the invention. The embodiment was chosen and described in order tobest explain the principles of the invention and the practicalapplication, and to enable others of ordinary skill in the art tounderstand the invention for various embodiment with variousmodifications as are suited to the particular use contemplated.

What is claimed is:
 1. A computer program product for executing amachine instruction, the computer program product comprising: anon-transitory computer readable storage medium readable by a processingcircuit and storing instructions for execution by the processing circuitfor performing a method comprising: obtaining, by a processor, a machineinstruction for execution, the machine instruction being defined forcomputer execution according to a computer architecture, the machineinstruction comprising: at least one opcode field for specifying anopcode, the opcode defining a branch operation, wherein the machineinstruction is a branch machine instruction; at least one field fordetermining a target location in which to branch; and a type indicationto specify a type of the branch operation, wherein the type indicationspecifies how the branch operation is to be processed in this executionof the machine instruction, wherein in another execution of the samebranch operation it is processed differently, and wherein the typeindication is configured to specify the following types of execution: aminor branch in which the machine instruction is executed in a firststream of processing and the branch operation does not cause a secondstream of processing to be started, a major branch open in which themachine instruction is executed in the first stream of processing andthe branch operation causes the second stream of processing to bestarted, and a major branch close in which processing of the firststream of processing meets up with processing of the second stream ofprocessing; and executing the branch machine instruction, the executingbeing based on the type indication specifying the major branch open, thebranch machine instruction being executed in the first stream ofprocessing, the executing comprising: starting the second stream ofprocessing, the second stream of processing having a relationship withthe first stream of processing and sharing hardware resources of thefirst stream of processing, wherein architectural state of the firststream of processing is referenced by the second stream of processing;determining, based on starting the second stream of processing, whethera transaction is to be started for the second stream of processing, thetransaction being defined as having results of instructions that areexecuted as part of the transaction being committed as one atomic unit,and wherein the determining comprises checking a field of the branchmachine instruction to determine whether a start of the transaction isindicated; based on determining that the transaction is to be started,starting the transaction for the second stream of processing, thetransaction to include one or more instructions beginning at the targetlocation defined using the at least one field of the branch machineinstruction, the one or more instructions being out of sequentialexecution order from one or more instructions being processed by thefirst stream of processing, the first stream of processing continuing toprocess one or more instructions subsequent to the branch machineinstruction, and wherein results of the one or more instructions of thesecond stream of processing executed as part of the transaction arecommitted as one atomic unit, and wherein the transaction does not enduntil the first stream of processing reaches a predefined point inprocessing and the second stream of processing is to become the firststream of processing; wherein execution of the second stream ofprocessing is speculative in that at least one instruction of the one ormore instructions of the second stream of processing is executed eventhough reaching the at least one instruction of the second stream ofprocessing by processing of the first stream is tentative; and obtainingby the processor another machine instruction, the other machineinstruction having the type indication, the type indication of the othermachine instruction specifying the major branch close indicatingprocessing of the first stream of processing has caught up with thetarget of the major branch open, and wherein execution of the majorbranch close comprises ending the transaction of the second stream ofprocessing, making the second stream of processing the first stream ofprocessing, and updating state to reflect the transaction.
 2. Thecomputer program product of claim 1, wherein the field of the branchmachine instruction used to determine whether the transaction is to bestarted is one of the following: the opcode, a part of the opcode, and afield separate from the opcode.
 3. The computer program product of claim1, wherein the determining comprises: determining, by hardware, whetherthe transaction is to be started; and wherein the starting thetransaction is based on the determining by hardware indicating that thetransaction is to be started.
 4. The computer program product of claim3, wherein the determining by hardware overrides the field of the branchmachine instruction indicating whether the transaction is to be started.5. The computer program product of claim 1, wherein at least oneinstruction of the one or more instructions processed in the secondstream of processing is processed, at least in part, in parallel to oneor more instructions executed in the first stream of processing.
 6. Thecomputer program product of claim 1, wherein the first stream ofprocessing is deallocated subsequent to the first stream of processingreaching an instruction having a target location of an instruction ofthe second stream of processing.
 7. The computer program product ofclaim 6, wherein the instruction reached by the first stream ofprocessing is the major branch close instruction.
 8. The computerprogram product of claim 1, wherein the at least one instruction of theone or more instructions of the second stream of processing is decodedand dispatched out-of-order with respect to at least one instruction ofthe first stream of processing.
 9. The computer program product of claim8, wherein the at least one instruction of the one or more instructionsof the second stream of processing is also executed out-of-order withrespect to at least one instruction of the first stream of processing.10. A computer system for executing a machine instruction, the computersystem comprising: a memory; and a processor in communications with thememory, wherein the computer system is configured to perform a method,said method comprising: obtaining, by the processor, a machineinstruction for execution, the machine instruction being defined forcomputer execution according to a computer architecture, the machineinstruction comprising: at least one opcode field for specifying anopcode, the opcode defining a branch operation, wherein the machineinstruction is a branch machine instruction; at least one field fordetermining a target location in which to branch; and a type indicationto specify a type of the branch operation, wherein the type indicationspecifies how the branch operation is to be processed in this executionof the machine instruction, wherein in another execution of the samebranch operation it is processed differently, and wherein the typeindication is configured to specify the following types of execution: aminor branch in which the machine instruction is executed in a firststream of processing and the branch operation does not cause a secondstream of processing to be started, a major branch open in which themachine instruction is executed in the first stream of processing andthe branch operation causes the second stream of processing to bestarted, and a major branch close in which processing of the firststream of processing meets up with processing of the second stream ofprocessing; and executing the branch machine instruction, the executingbeing based on the type indication specifying the major branch open, thebranch machine instruction being executed in the first stream ofprocessing, the executing comprising: starting the second stream ofprocessing, the second stream of processing having a relationship withthe first stream of processing and sharing hardware resources of thefirst stream of processing, wherein architectural state of the firststream of processing is referenced by the second stream of processing;determining, based on starting the second stream of processing, whethera transaction is to be started for the second stream of processing, thetransaction being defined as having results of instructions that areexecuted as part of the transaction being committed as one atomic unit,and wherein the determining comprises checking a field of the branchmachine instruction to determine whether a start of the transaction isindicated; based on determining that the transaction is to be started,starting the transaction for the second stream of processing, thetransaction to include one or more instructions beginning at the targetlocation defined using the at least one field of the branch machineinstruction, the one or more instructions being out of sequentialexecution order from one or more instructions being processed by thefirst stream of processing, the first stream of processing continuing toprocess one or more instructions subsequent to the branch machineinstruction, and wherein results of the one or more instructions of thesecond stream of processing executed as part of the transaction arecommitted as one atomic unit, and wherein the transaction does not enduntil the first stream of processing reaches a predefined point inprocessing and the second stream of processing is to become the firststream of processing; wherein execution of the second stream ofprocessing is speculative in that at least one instruction of the one ormore instructions of the second stream of processing is executed eventhough reaching the at least one instruction of the second stream ofprocessing by processing of the first stream is tentative; and obtainingby the processor another machine instruction, the other machineinstruction having the type indication, the type indication of the othermachine instruction specifying the major branch close indicatingprocessing of the first stream of processing has caught up with thetarget of the major branch open, and wherein execution of the majorbranch close comprises ending the transaction of the second stream ofprocessing, making the second stream of processing the first stream ofprocessing, and updating state to reflect the transaction.
 11. Thecomputer system of claim 10, wherein the field of the branch machineinstruction used to determine whether the transaction is to be startedis one of the following: the opcode, a part of the opcode, and a fieldseparate from the opcode.
 12. The computer system of claim 10, whereinthe determining comprises: determining, by hardware, whether thetransaction is to be started; and wherein the starting the transactionis based on the determining by hardware indicating that the transactionis to be started.
 13. The computer system of claim 12, wherein thedetermining by hardware overrides the field of the branch machineinstruction indicating whether the transaction is to be started.
 14. Thecomputer system of claim 10, wherein the at least one instruction of theone or more instructions of the second stream of processing is decodedand dispatched out-of-order with respect to at least one instruction ofthe first stream of processing.
 15. The computer system of claim 14,wherein the at least one instruction of the one or more instructions ofthe second stream of processing is also executed out-of-order withrespect to at least one instruction of the first stream of processing.16. A computer program product for executing a machine instruction, thecomputer program product comprising: a non-transitory computer readablestorage medium readable by a processing circuit and storing instructionsfor execution by the processing circuit for performing a methodcomprising: obtaining, by a processor, a machine instruction forexecution, the machine instruction being defined for computer executionaccording to a computer architecture, the machine instructioncomprising: at least one opcode field for specifying an opcode, theopcode defining a branch operation, wherein the machine instruction is abranch machine instruction; a type indication to specify a type of thebranch operation, wherein the same branch operation is executable in aplurality of different ways depending on the type indication, whereinthe type indication specifies how the branch operation is to beprocessed in this execution of the machine instruction, and wherein thetype indication is configured to specify the following types ofexecution: a minor branch in which the machine instruction is executedin a first stream of processing and the branch operation does not causea second stream of processing to be started, a major branch open inwhich the machine instruction is executed in the first stream ofprocessing and the branch operation causes the second stream ofprocessing to be started, and a major branch close in which processingof the first stream of processing meets up with processing of the secondstream of processing; and at least one field for determining a targetlocation in which to branch; and executing the branch machineinstruction, the executing being based on the type indication specifyingthe major branch open, the branch machine instruction being executed inthe first stream of processing, the executing comprising: starting thesecond stream of processing, the second stream of processing having arelationship with the first stream of processing and sharing hardwareresources of the first stream of processing, wherein architectural stateof the first stream of processing is referenced by the second stream ofprocessing; determining, based on starting the second stream ofprocessing, whether a transaction is to be started for the second streamof processing, the transaction being defined as having results ofinstructions that are executed as part of the transaction beingcommitted as one atomic unit, and wherein the determining compriseschecking a field of the branch machine instruction to determine whethera start of the transaction is indicated; based on determining that thetransaction is to be started, starting the transaction for the secondstream of processing, the transaction to include one or moreinstructions beginning at the target location defined using the at leastone field of the branch machine instruction, the one or moreinstructions being out of sequential execution order from one or moreinstructions being processed by the first stream of processing, thefirst stream of processing continuing to process one or moreinstructions subsequent to the branch machine instruction, and whereinresults of the one or more instructions of the second stream ofprocessing executed as part of the transaction are committed as oneatomic unit, and wherein the transaction does not end until the firststream of processing reaches a predefined point in processing and thesecond stream of processing is to become the first stream of processing;and wherein execution of the second stream of processing is speculativein that at least one instruction of the one or more instructions of thesecond stream of processing is executed even though reaching the atleast one instruction of the second stream of processing by processingof the first stream is tentative.